1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and, more particularly, an U electrophotographic photosensitive member having an improved intermediate layer provided between a substrate and a photosensitive layer.
2. Description of the Related Art
One of the significant requirements for an electrophotographic photosensitive member is that it possesses satisfactory charging characteristics, such as stability of dark and bright potentials against repeated charging-exposure cycles, uniformity of sensitivity and the like.
Therefore, it has been proposed to provide an intermediate layer or layers having various functions between the substrate and the photosensitive layer: such intermediate layers include a barrier layer which prevents injection of charges from the substrate into the photosensitive layer, a bonding layer for improving the strength of bonding between the substrate and the photosensitive layer, and a cladding layer having a cladding function to cover any defect in the substrate for the purpose of obtaining a photosensitive layer of uniform thickness.
For instance, hitherto, intermediate layers of the following materials have been proposed in the following Japanese Unexamined Patent Publications: polyamide (Japanese Unexamined Patent Publication Nos. 46-47344 and 52-25638); polyester (Japanese Unexamined Patent Publication Nos. 52-20836 and 54-26738); polyurethane (Japanese Unexamined Patent Publication Nos. 49-10044 and 53-89435); casein (Japanese Unexamined Patent Publication No. 55-103556); polypeptide (Japanese Unexamined Patent Publication No. 53-48523); polyvinyl alcohol (Japanese Unexamined Patent Publication No. 52-100240); polyvinyl pyrrolidone (Japanese Unexamined Patent Publication No. 48-30936);vinyl acetate-ethylene copolymer (Japanese Unexamined Patent Publication No. 48-26141); maleic anhydride ester copolymer (Japanese Unexamined Patent Publication No. 52-10138); polyvinyl butyral (Japanese Unexamined Patent Publication Nos. 57-90639 and 58-106549); polymer containing tetra ammonium salt (Japanese Unexamined Patent Publication Nos. 51-126149 and 56-60448); and ethyl cellulose ((Japanese Unexamined Patent Publication No. 55-143564).
However, these known electrophotographic photosensitive members having intermediate layers of the above-mentioned materials could not provide stable potential characteristics and image quality over a wide environmental condition including low-temperature and low-humidity condition and high-temperature and high-humidity condition, because the electrical resistance of the intermediate layer significantly varies according to the temperature and humidity.
For instance, when the photosensitive member is used repeatedly in an environment of low temperature and low humidity in which the intermediate layer exhibits a greater resistance, the bright potential and the residual potential rises due to the presence of residual charges on the intermediate layer, with the result that fog is generated on the copied image. When such a photosensitive member is used in an electrophotographic printer which employs inversion development, the density of the image is reduced or the quality of copies fluctuates undesirably.
When this type of photosensitive member is used in an environment of high temperature and high humidity, the barrier function is impaired due to a change in the resistance of the intermediate layer so that the injection of carriers from the substrate is enhanced, which reduces the bright potential. Consequently, the thickness or density of the copy image is reduced at high temperature and humidity. When such a photosensitive member is used in an electrophotographic printer which utilizes inversion development, the copy images tend to be impaired by black spots and fog.